When creating two volumes adjacent to each other, do they need to share a common face? I see that I can make two (or more) faces at the same location, which can be used to construct different volumes. I would not like to boolean add these volumes together because that makes me lose grid information on some of the edges. Which way should I go to make sure that the face is a fluid-fluid interface -- one face or two?

If two volumes share one common face you can forget about it...fluent(gambit) automatically sets it as a fluid-fluid interface. If you have 2 faces (each designated to one volume) in the same position (for example if you create 2 cubes on besides another) fluent(gambit) will treat it as an outside wall...that means that yoou have to specify them (both!) as intrefaces...